The present invention relates to a noise reduction system and for a passenger compartment of automotive vehicle and method of positively generating a sound from a sound source to cancel the vehicle internal noise.
Japanese Patent Application No. Toku-Kai-Hei 3-178846 discloses a noise reduction technique for reducing noise by means of controlling a tap value of an adaptive filter to generate a canceling sound with a reversed phase by an output of a microphone which is disposed at a position where a noise is to be reduced and by a signal to compensate a transmission characteristic of the propagation path of a signal from the adaptive filter to the microphone,
The noise reduction apparatus of the above prior art makes an adaptive control by recording in a transmission characteristic compensation section the transmission characteristics of a propagation path where the speaker transforms a signal from the adaptive filter into a sound wave and the sound wave propagates from the speaker to the microphone.
In the noise reduction system according to the prior art there is a disadvantage that in case where noises generated from the noise source contain frequency components which do not exist in the frequency characteristic of the speaker of the system, the noise reduction becomes unstable. In this case, the adaptive filter performs an adaptive control and transmits a canceling signal for canceling these noises out of the frequency band of the speaker.
However, the canceling signal generated from the adaptive filter is cut by the speaker and is not propagated into the space. Therefore, the error signal of the frequency component out of the band is feedback in spite of performing an adaptive control.
Because of this feedback error signal, the adaptive filter operates so as to output a signal with further larger amplitude, and as a result a distortion is caused by a fluctuation of the amplifier driving the speaker, and when it goes worse the adaptive control system would be diverged.